Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, touch sensor panels, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface. The touch sensor panel can be positioned in front of a display screen so that the touch-sensitive surface covers the viewable area of the display screen. Touch screens can allow a user to make selections, move a cursor, or perform drawing operations, to name just a few examples, by simply touching the display screen via a finger or stylus. In general, the touch screen can recognize a touch and position of the touch on the display screen, and the computing system can interpret the touch and perform an action based on the touch event.
One or more stimulation signals can be applied to the drive lines of a mutual capacitance touch sensor panel, and these signals can be capacitively coupled onto the sense lines. Alternatively, the stimulation signals can be applied to the sense electrodes of a self-capacitance touch sensor panel. In either situation, the amplitude of the sense signals appearing on the sense lines or sense electrodes can be affected by an object touching or in proximity to the touch sensor panel. In order to properly detect the presence and location of the object, the amplitude of the sense signals should be large enough that the variations in the sense signals due to the object can be accurately detected and located even in the presence of noise. In other words, the signal-to-noise ratio (SNR) should be maintained at a certain level for acceptable touch performance. To accomplish this, conventional touch sensing systems operate at a fixed stimulation signal voltage independent of power requirements, and also regardless of SNR margin. The fixed stimulation voltage is often chosen to ensure proper operation even in a worst-case noise situation. However, by operating to worst-case conditions, in most environments without worst-case noise there can be excess SNR and unnecessary power consumption.